When most people think of the Amazon
rainforest, they envision a vast expanse of green jungle, dripping with
moisture year round. Some parts of the Amazon do receive relentless
rain, but other regions have a distinct dry season when little or no
rain falls.

The southeastern Amazon, in the Brazilian states of Mato Grosso and
Pará, has a four-to-five month dry season that limits rains to a
trickle. Lush vegetation survives by sending roots up to 10 meters (30
feet) into the ground. Ecologists worry that as climate change
brings hotter, drier conditions to the region, the southern edge of the
Amazon could easily transition from forest to tropical savanna known in
South America as the Cerrado.

The boundary between rainforest and the Cerrado cuts across the
Amazon Basin in a ragged line that runs through the Brazilian states of
Rondônia, Mato Grosso, and Pará. The boundary has emerged as one of the
most interesting parts of the Amazon to study because it is at this
forest frontier where the needs of modern Brazilian society—cleared land
for farmers, ranchers, villages, and roads—come into direct conflict
with those of the rainforest and the large populations of indigenous
peoples who live within the forest.

Then there are the fires. Satellite and ground observations of the forest edge show evidence of what ecologists call “understory” fires—low-intensity
blazes that creep along the leaf littler on the forest floor but never
get hot enough to burn up through the canopy. Since the fires remain
beneath the canopy, NASA satellites designed to monitor fire activity
rarely detect understory fires (unlike those that release more energy,
such as deforestation or agricultural management fires). But satellites can
sense the subtle changes in the forest canopy in the wake of these
understory fires, as the damaged vegetation slowly dies and new trees
grow back.

Distinguishing between understory fire damage and other types of forest disturbance—such as selective logging and deforestation
for agriculture—is not straightforward with satellite imagery,
explained NASA scientist Doug Morton. In the last few years, Morton has
worked out a new technique based on data from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor that makes it easier to distinguish understory fires from similar-looking forest disturbances.

The technique has made it possible for Morton and his colleagues to map the distribution
of understory fires across the entire southern Amazon basin for the
first time. (See the large image for the distribution of understory
fires across the basin). Morton found that understory forest fires
burned more than 85,500 square kilometers (33,000 square miles), or 2.8
percent of the forest, between 1999 and 2010.

The prevailing theory has been that deforestation and understory
fires go hand in hand, because deforestation creates gaps in the
rainforest canopy that dry out the forest floor. Surprisingly, Morton’s
mapping found no correlation between understory fires and deforestation.
In fact, even in areas with little or no deforestation—such as the
large reserves in the Xingu River Basin set aside for Brazil’s indigenous people—the analysis detected very large understory fires, with some areas burning frequently.

The Xingu River Basin saw more forests with repeated burning than
nearly anywhere else in the Amazon over the study period, with some
areas facing four or more fires. As shown in the map above, the fires
within the indigenous reserve were concentrated along the banks of the
Xingu River; rivers are the main transportation network for indigenous
groups in the Xingu basin, similar to other remote regions of the Amazon
where roads are scarce or seasonally flooded. Amongst the farms and
ranches on the forest frontier beyond the reserve, fires were also
common but scattered more broadly.

“One of the things this research shows is that deforestation probably
isn’t the key driver of understory fires we once thought it was,” said
Morton. “The simple presence of people—indigenous or otherwise—along
with hot, dry weather is all you need for large understory fires to burn
along the southern edge of the Amazon.”

Morton’s next question: “Are the forests along the edges of the
Amazon able to withstand all these understory fires or will we see a
gradual transition from forest to savanna over time?”